DESCRIPTION (Applicant's Description-edited) A commonly (mis)quoted statistic on female breast cancer is that 75 percent of cases have no known risk factors. This statistic often supplements other data to gain public support for increased etiologic research. In addressing the breast cancer etiology priority area of the NAPBC RFA, this research will provide updated, appropriate and clarified estimates of the proportion of breast cancer due to known or suspected risk factors. These estimates will be useful to breast cancer constituencies such as scientists, researchers, advocates, survivors, and educators. This research may also have relevance to the information dissemination priority. The research focuses on breast cancer attributable risk, commonly defined as the proportion of cases which are explained by a set of one or more known risk factors. A low attributable risk indicates that the salient risk factors have not been identified. The two most commonly quoted estimates for breast cancer attributable risk are .25 (Seidman et al, 1982) and .55 (Bruzzi et al, 1985). The .25 figure frequently is misinterpreted as indicated above. In fact, Seidman's data show that about 80 percent of breast cancer cases have at least one of the risk factors. Not only do the Seidman and Bruzzi estimates of attributable risk differ by a factor of 2 because of different methodology, but they are for white women only, are based on older data from the 1960's and 1970's which do not reflect women's more recent experience, and frequently are misinterpreted. We will use more recent data from two prospective studies (CPS-II and BCDDP follow-up) and one case-control study (WISH) to provide more recent estimates of breast cancer attributable risk, using the Bruzzi and Seidman methodology with needed modifications as well as other more recently developed estimation methods. The newer estimates might be higher or lower, depending upon shifts in relative risk and prevalence of known risk factors. The research product will be updated summary measures of breast cancer attributable risk for white, black and total women and for younger and older women. These summary measures will help assess the current state of breast cancer etiologic research. Dissemination of results, carefully explained and interpreted, will go beyond submission of manuscripts to journals to include major breast cancer constituencies.